For biaxial stretching blow molding of a preform as an injection molded product made of the PET into a bottle shape, it is necessary to heat the preform to a temperature at which it can be stretched. As a method of heating the preform, there are a method of heating it by a heat transfer from a temperature of an atmosphere and a method of heating it by absorption of light.
According to the method of heating the preform by the heat transfer from the atmosphere temperature, since a coefficient of thermal conductivity of the PET material is slightly lower, it takes a long time to heat the whole preform including an inner surface to a desired temperature. On the other hand, according to the method of heating the preform by the light absorption, since the preform is heated by the light of a wavelength which penetrates into the inner surface of the preform, a time which is necessary for heating the whole preform including the inner surface to the desired temperature or higher is shorter than that of the heating means by the heat transfer mentioned above.
As mentioned above, upon heating of the preform at the time of the biaxial stretching blow molding of the bottle, as disclosed in JP-A-11-42702, an infrared lamp (near infrared heater) for generating near infrared rays is generally used as a heat source. A plurality of near infrared heaters are arranged in parallel along the axial center direction of the conveyed preform and a power of each near infrared heater is adjusted, thereby heating the preform in accordance with a desired temperature distribution along the axial center.
However, even if the near infrared heaters are used as heating sources of the preform or another proper heat source (for example, a sheath heater or the like) is used, since the preform is heated from its outside, the following problem exists. That is, a temperature rising speed on the outer surface side of the preform is certainly higher than that on the inner surface side, a temperature difference occurs between the outside and the inside of the preform which is heated, if the temperature difference is large, the proper biaxial stretching blow molding operation cannot be performed, so that it is necessary to limit the power of the heat sources in order to suppress the heating temperature difference between the inner surface side and the outer surface side of the preform to a predetermined value or less. Thus, heating time of the preform cannot be sufficiently shortened.
A technique using both of near infrared heating means and dielectric heating means in order to shorten the heating time of the preform has been disclosed (refer to JP-A-8-142175). Such a technique, however, has the following problem. That is, since both of those means have to be made operative almost simultaneously, the heating time cannot be sufficiently shortened. Further, since heating equipment is large in size and complicated, a large installing space and the expensive cost of equipment are necessary.
The invention is, therefore, made to solve the problems in the prior arts as mentioned above and it is an object of the invention to realize a technical subject which enables heat sources to operate with a high power upon heating of a preform and to thereby accomplish a large reduction of heating time of the preform and miniaturization of a whole preform heating device.